High power, narrow linewidth laser radiation is frequently generated in a master oscillator power amplifier (MOPA) configuration that utilizes a low power, single frequency seed laser, followed by a high power fiber amplifier. These sources are currently under development for a variety of applications, such as free-space optical communications (FSOC), range finding, spectroscopy and interferometry. The maximum achievable power in a conventional fiber amplifier is typically limited by the onset of stimulated Brillouin scattering (SBS). SBS is an inherent effect that occurs in fiber amplifiers in which the forward-propagating power in the amplifier is converted into backward propagating power with a slightly downward frequency shift that limits the power transfer through a fiber amplifier. SBS gain is dependent on beam intensity, spectral width, and fiber length, generating both forward and backward propagating acoustic phonons at the Brillouin shift frequency, defined as:νB=2·VS·n/λ0,where n is the approximate refractive index of the fiber core, λ0 the laser wavelength in a vacuum and VS is the approximate speed of sound in the fiber core. It should be recognized that SBS does not generally occur at low powers and for short lengths of optical fiber over which a signal is transmitted.
In high power fibers, there is a desire for relatively small modal diameters (on the order of approximately 6 μm for single mode fibers and 25 μm for multimode fibers), source linewidths less that the SBS gain-bandwidth (approximately 50 MHz) and relatively long fiber lengths (on the order of 10 m). The presence of SBS in high power, narrow linewidth lasers causes several problems: (1) it limits the maximum output power of the amplifier; (2) the reflected power experiences further amplification by the ion inversion in the gain fiber, thus generating “giant” pulses with very high peak powers that can damage the optical fiber, and (3) the reflected optical power can undergo secondary reflections so that extraneous pulses can appear on the fiber amplifier output, corrupting temporal and spectral characteristics of the output pulse.
Thus, a need remains in the art for an arrangement that mitigates the presence of SBS in high power, narrow linewidth fiber amplifiers.